1. Field of the Invention
The present invention relates to a digital signal selective or detecting circuit for providing an output signal in response to a periodic input signal which has a frequency within a specified range for a predetermined time. In particular, the present invention relates to a digital signal selective circuit suitable for use with a video tape recorder (VTR) and responsive to a cue signal recorded on a video tape used in the VTR.
2. Description of the Prior Art
Frequency detecting circuits have been proposed for detecting an indication or command signal which maintains a predetermined frequency for a predetermined length of time in order to initiate the operation of various associated systems. For instance, such frequency detecting circuits are commonly found in such widely ranging applications as remotely controlled automatic garage door openers and automatic telephone answering machines. However, prior art frequency detecting circuits employ analog rather than digital techniques. A typical analog frequency detecting circuit incorporates a resonance filter, such as a tank circuit, a reed filter, an electromechanical filter, or a time-constant circuit. In such analog circuits, detection errors often occur because of detector instability whenever an incoming waveform is distorted. In addition, it is difficult to adjust the alignment and bias of the various elements in an analog circuit. Further, in prior art analog frequency detecting circuits, it is difficult to change the response or detected frequency.